Fluid transportation system

ABSTRACT

Method and apparatus for assisting the flow of production fluid from a hydrocarbon well to a remote location in conditions in which gelling or solidification is a problem. The method involves adding dilution fluid ( 60 ) such as water, to production fluid from a wellhead ( 4 ) in a first sub-system ( 8 ) close to the wellhead ( 4 ), conveying the mixture to a second sub-system ( 14 ) where the dilution fluid ( 60 ) is separated from the mixture in a separator chamber ( 38 ) as a consequence of their different specific gravities, recirculating the separated dilution fluid back to the first sub-system ( 8 ) and adding it to further production fluid from the wellhead ( 4 ). The requirements for pipeline heating, chemical injection and conveying large volumes of dilution fluid to a host facility can be avoided by the invention.

[0001] The present invention relates to a method and apparatus forassisting a flow of hydrocarbon fluid from a wellhead of a hydrocarbonextraction well to a remote location.

[0002] When production fluid from such a well is highly viscous and/orexhibits non-Newtonian rheology (i.e. non-linear relationship betweenrate of deformation and applied shear stress), which may also be thecase when the fluid contains wax, there is a tendency for the fluid togel or even solidify during transportation through a pipeline runningfrom the wellhead to a remote location such as a host facility. Thisoccurs because the temperature of the fluid falls once it leaves thewellhead causing its viscosity to increase. This problem is particularlypronounced when the pipeline runs along the sea bed where temperaturesare low. In the past, this problem has been addressed by a variety oftechniques. A first technique is to insulate the pipeline which iscostly since the pipeline may be tens of kilometres long. Furthermore,in the event of shutdown occurring for any reason, it is necessary toinject chemicals into the production fluid to prevent gelling orsolidification occurring. A second technique is to rely solely oninjected chemicals. Any injection of such chemicals suffers from thedisadvantages that appropriate chemicals need to be purchased and storedat the host facility. Also a dedicated chemical injection pipelineleading from the host facility to the wellhead needs to be provided aswell as equipment at the host facility for recovering the chemicals fromthe production fluid.

[0003] A third technique is to use a so-called “pipe-in-pipe” forconveying the production fluid from the wellhead to the host facility.With this technique a continuous flow of heated water is passed throughan outer pipe in which an inner production fluid transportation pipe issituated. The capital cost of the pipeline is high as are the runningcosts associated with continuously providing heated water, which isgenerally discharged into the sea at the wellhead. As explained above,the injection of appropriate chemicals is required in the event ofshutdown occurring. Heating may alternatively be provided by traceheaters in the pipeline in combination with insulation over the majorityof its length

[0004] A fourth technique is to continuously pump water from the hostfacility down a high pressure riser and pipeline to the wellhead (or toa down hole location) where the water is combined with the productionfluid producing a mixture which is conveyed to the host facility whereseparation occurs. This technique suffers from the significant costsassociated with (a) providing a high pressure pipeline for deliveringdilution fluid to the wellhead; (b) providing equipment at the hostfacility to treat the water so that it is suitable for mixing with theproduction fluid; (c) pumping water at high pressure to the wellhead;and (d) separating the water from the production fluid once it hasreturned to the host facility. Furthermore, in a situation in which thehost facility is owned by a company which is different from thateffecting extraction, a levy will generally be paid to the host ownerwhich is dependent on the overall volume of fluid received by the hostfacility. Increasing this overall volume by the addition of dilutionfluid adds to the levy payable. A charge is also likely to be levied forwater provided by the host facility.

[0005] An object of the invention is to provide a system which overcomesat least some of the problems of the prior art discussed above.

[0006] Thus according to a first aspect of the invention there isprovided a method of assisting flow of production fluid from a sub-seawellhead to a remote sub-sea location including the steps of: (a) addingdilution fluid to the production fluid at a dilution fluid additionlocation close to or at the wellhead to provide a mixture; (b) conveyingthe mixture through a pipe to the remote location; (c) separating atleast some of the dilution fluid from the mixture at the remotelocation; (d) conveying the separated dilution fluid from the remotelocation to the dilution fluid addition location; and (e) adding theseparated dilution fluid at the dilution fluid addition location tofurther production fluid flowing through the wellhead.

[0007] Such a method avoids the requirement to take any particularprecautionary measures if shutdown is to occur. This is particularlyuseful since it may not be possible to anticipate such a shutdown farenough in advance. Furthermore, the use of chemicals and the requirementfor a dedicated chemical injection line to the wellhead can be avoidedand a requirement for a continuous supply of treated water pumped underhigh pressure from the host facility can also be avoided. The inventivemethod also avoids the high capital expenditure of pipe-in-pipe linesand pipes with electrical heating elements. The running costs associatedwith heating a continuous supply of diluting fluid, such as water, orproviding a continuous electricity supply for electrical heating of thepipe are also avoided.

[0008] The tariff paid to a host operator will not be increased as adirect or indirect result of delivering a mixture of production fluidand a large amount of diluting fluid (normally water) to the hostfacility.

[0009] Preferably the remote sub-sea location is situated close to ahost facility so that after separation of the dilution fluid from theproduction fluid, the production fluid only needs to be transported overa short distance.

[0010] In order to be able to take water from the host facility, whichwould not be able to be discharged directly into the sea, preferablyprior to step (a) the dilution fluid is pumped from the host facility tothe dilution fluid addition location possibly via the remote location.Such a step may well attract a negative tariff for a company using thehost facility (i.e. the host facility owner pays the user for usingcontaminated water from the host).

[0011] The method preferably also involves the step of conveying theseparated production fluid to the host facility.

[0012] Conveniently the dilution fluid essentially comprises water andthe volume ratio of the dilution fluid to the production fluid in themixture may lie in the range 2:1 to 4:1 and more preferably in the range2.5:1 to 3.5:1. Such a ratio provides adequate dilution without undulyincreasing the volume of the mixture to be conveyed to the remotelocation.

[0013] To increase efficiency of the method, the step of separating thedilution fluid from the mixture preferably involves separating amajority of the dilution fluid therefrom and more preferably separatingat least 90% of it therefrom.

[0014] Preferably the separation of the dilution fluid from the mixtureinvolves routing the mixture into a separator chamber of a system at theremote location where separation of the dilution fluid from theproduction fluid occurs as a consequence of their different specificgravities. The equipment for effecting such separation can be simple androbust and suitable for operating in an underwater location.

[0015] Preferably the system at the remote location includes a pump andthe conveying of the separated dilution fluid to the dilution fluidaddition location includes pumping the dilution fluid with the pump.

[0016] So as to increase the credit provided for removing contaminatedwater from the host facility, preferably the method includes conveyingsome of the water which has been used as dilution fluid to a disposalwell. The method may also involve pumping a certain amount ofcontaminated water directly from the host to the disposal well, possiblyvia the remote location. When such a supply to a disposal well isemployed, preferably a pump is situated at the dilution fluid additionlocation and the step of conveying the dilution fluid (e.g. water) tothe disposal well includes pumping it there with the pump.

[0017] According to a second aspect of the invention there is providedapparatus for assisting flow of production fluid from a sea-bed wellheadto a remote sub-sea location including dilution fluid addition meanssituated close to or at the wellhead for adding dilution fluid to theproduction fluid to produce a mixture, a pipe for conveying the mixtureto the remote location, separating means at the remote location forseparating at least some of the dilution fluid from the mixture andmeans for conveying the separated dilution fluid from the remotelocation to the dilution fluid addition means for addition to furtherproduction fluid flowing through the wellhead.

[0018] The invention will now be described by way of example only withreference to the accompanying schematic Figures in which:

[0019]FIG. 1 shows a system for putting the invention into practice;

[0020]FIG. 2 shows details of two seabed sub-systems of the system shownin FIG. 1;

[0021]FIG. 3 shows a modified system for putting the invention intopractice; and

[0022]FIG. 4 shows details of the two seabed sub-systems of the systemshown in FIG. 3.

[0023] The system of FIG. 1 shows a host facility 2 connected to receiveproduction fluid from a wellhead tree 4. The production fluid will bereferred to below as oil but may be a mixture of fluids such as oil andgas. The wellhead tree 4 is connected by an output pipe 6 to a firstseabed sub-system 8 which is connected by a mixture pipeline 10 and adilution fluid pipeline 12 to a second seabed sub-system 14 situatedremotely therefrom. While the sub-systems have been described as seabedsub-systems they may be floating and/or tethered to the seabed. Each ofthe first and second sub-systems are respectively positioned close tothe wellhead tree 4 and the host facility 2 relative to the distancethey are apart from each other. The second sub-system 14 is connected tothe host facility 2 by means of a production fluid riser 16 and adilution fluid supply riser 18.

[0024] The components in each of the sub-systems 8 and 14 will now bedescribed in detail with reference to FIG. 2.

[0025] The first sub-system 8 has a production inlet 20 connected to theoutput pipe 6, a dilution fluid inlet 22 connected to the dilution fluidpipeline 12 and mixture outlet 24 connected to the mixture pipeline 10.A mixing loop pipe 26 connects the dilution fluid inlet 22 to anintermediate junction 29 on a production conduit 28 extending betweenthe production inlet 20 and the mixture outlet 24. The mixing loop 26has a remotely actuable throttle valve 30 and a flow meter 31 forrespectively controlling and measuring flow through the mixing loop 26.

[0026] The second sub-system 14 has duplicated first and secondseparations systems 32 and 34, only the first 32 of which will bedescribed in detail. A mixture inlet 36 is connected to the mixturepipeline 10 and to an inlet 70 of a separator chamber 38 via a failsafevalve 40. A first outlet 44 of the chamber 38 is connected to a dilutionfluid outlet 46 via a controlling throttle valve 48 and non-return valve50. A second outlet 52 of the chamber 38 is connected to a separatedproduction fluid outlet 54 via a controlling throttle valve 56. Theseparated production fluid outlet 54 is connected to the productionfluid riser 16. A level sensor 42 senses the position of the interfacebetween dilution fluid 60 and oil 63 in the chamber 38 and a pressuresensor 58 senses the pressure in the chamber. A weir 61 is situatedbetween the first and second chamber outlets 44 and 52.

[0027] A pressure boosting pump 62 is positioned in a conduit 68 leadingfrom the first chamber outlet 44 to the dilution fluid outlet 46 forpumping dilution fluid out of the chamber 38. The pump 62 is designed tobe capable of pumping the dilution fluid into the flow of produced fluidat the junction 29. An additional pump may be situated in the mixingloop to facilitate this process. Where it is necessary to boost thepressure of the production fluid, to enable it to reach the secondsub-system at an appropriate flow rate, a jet pump may be situated atjunction 29 arranged so that the dilution fluid is the diving fluidwhich entrains production fluid. A recirculation loop pipe 64 connectsthe conduit 68, downstream of the recirculation pump 62, to the inlet 70of the chamber 38 via a non-return valve 66 and a restricting device 67such as an orifice plate. The purpose of the restricting device 67 is toensure that dilution fluid does not merely take the path of leastresistance and be short circuited through the separator.

[0028] The dilution fluid supply riser 18, leading from the hostfacility, is connected to a dilution fluid inlet 72 which is connectedby a dilution fluid conduit 74, containing a non-return valve 76, to thedilution fluid outlet 46.

[0029] The host facility 2 includes processing equipment (not shown) forprocessing production fluid received from the production fluid riser 16and providing a supply of treated dilution fluid, for example water,which is suitable for dilution of the production fluid and may be pumpeddown the dilution fluid supply riser 18 to the dilution fluid inlet 72of the second sub-system 14.

[0030] The manner in which the system operates will now be described.The description will refer to the dilution fluid as being water althoughit could be an alternative fluid.

[0031] If the viscosity of the oil emerging from the wellhead tree 4 istoo viscous, so that there is a danger of it gelling duringtransportation to the host facility, then a batch of dilution fluid(e.g. sea-water or water treated to prevent adverse reaction such asscale formation) may be supplied from the host facility down thedilution fluid supply riser 18 into the second sub-system 14 at thedilution fluid inlet 72, through the conduit 74 and out via the dilutionfluid outlet 46. The water then flows through the dilution fluidpipeline 12 to the dilution fluid inlet 22 of the first sub-system 8where it is routed via the mixing loop 26 to the junction 29 with theproduction conduit 28 where it mixes with the production fluid from thewellhead tree 4. Signals from the flow meter 31 are monitored by acontrol system (not shown) which controls the opening of the throttlevalve 30 so that water is mixed with the oil in the ratio 3:1 typically.The resulting mixture then leaves the first sub-system 8 at the mixtureoutlet 24 and is conveyed via the mixture pipeline 10 to the mixtureinlet 36 of the second sub-system 14 where it is routed into one of thetwo separating systems 32 and 34. In the chamber 38 of the separatingsystem, the water 60 occupies the region to the left of the weir 61 (asseen in FIG. 2) and oil floats above the water and passes over the weir61 into the region to the right thereof. Separated oil then flows out ofthe chamber 38 via the second outlet 52 to the production fluid outlet54 and up the production fluid riser 16 to the host facility 2. Theseparated water 60 flows from the chamber 38 through its first outlet 44and through the conduit 68 to the dilution fluid outlet 46. The water isthen returned from the second to the first sub-system where it entersthe first sub-system at the dilution fluid inlet 22 from where itbecomes mixed with further oil as described above.

[0032] The control system (not shown), on the basis of signals receivedfrom the level sensor 42 and pressure sensor 58, controls the operationof throttle valves 56 and 48, controlling the flows of oil and waterrespectively leaving the chamber 38, and the water pressure boostingpump 62 to maintain the fluid interface between the oil 63 and the water60 in the chamber 38 below the top of the weir 61. If there is arequirement to increase the amount of water in the chamber 38, thethrottle valve 48 would be closed to a certain extent so that waterwould be forced by the recirculation pump 62 through the pressureboosting loop 64 back into the chamber 38 rather than back to the firstsub-system 8.

[0033] Using the method described above, the same batch of water isrepeatedly used to dilute oil flowing through the mixture pipeline 10between the first and second sub-systems 8 and 14. Accordingly, acontinuous supply of water from the host facility is not required andoil reaching the host facility is substantially free of diluting waterwhich has been removed from the mixture at the second sub-system 14.

[0034]FIG. 4 shows a modified system in which parts which correspond tothose shown in FIG. 2 are designated with the same reference numeral andnot described in detail below.

[0035] The system shown in FIG. 4 differs from that shown in FIG. 2 inthat it includes a means for delivering fluid received from the hostfacility and/or the first outlets 44 of the chambers 38 in the secondsub-system to a deposal well 78.

[0036] The dilution fluid inlet 22 of the first sub-system is connectedby two pumps 80 to a disposal fluid outlet 82 which is connected by adisposal pipeline 86 to a wellhead tree of the disposal well 78. A flowmeter 84 is situated in a disposal conduit 88 constituting thisconnection for sensing the rate of flow to the disposal well 78. Twopumps are provided rather than one merely to provided redundancy so thatconveyance of fluid to the disposal well need not be interrupted if onepump is not operating for any reason.

[0037] When water from the host is to be disposed of directly withoutperforming any dilution function it will pass through the so-calleddilution pipeline 12 notwithstanding the fact that it is not being usedfor any dilution purposes.

[0038] Although the invention has been described in the context of asub-sea hydrocarbon field, it would also be applicable to fields inother environments in which access constitutes a problem, for example inswampy areas, and/or fields in cold climatic areas such as the Arctic.

1. A method of assisting flow of production fluid from a subsea wellhead(4) to a remote subsea location (14) including the steps of: (a) addingdilution fluid to production fluid at a dilution fluid addition location(8) close to or at the wellhead (4) to provide a mixture; (b) conveyingthe mixture through a pipe (10) to the remote location (14); (c)separating at least some of the dilution fluid from the mixture at theremote location (14); (d) conveying (12) the separated dilution fluidfrom the remote location (14) to the dilution fluid addition location(8); and (e) adding the separated dilution fluid to further productionfluid flowing through the wellhead (4) at the dilution fluid additionlocation (8).
 2. The method according to claim 1 wherein the remotesubsea location (14) is situated close to a host facility (2).
 3. Themethod according to claims 1 and 2 wherein prior to step (a) thedilution fluid is supplied from the host facility (2) to the dilutionfluid addition location (8).
 4. The method according to claim 2 or 3comprising the further step of conveying (16) the separated productionfluid to the host facility (2).
 5. The method according to any precedingclaim wherein the dilution fluid essentially comprises water.
 6. Themethod according to any preceding claim wherein the ratio of dilutionfluid to production fluid in the mixture lies in the range 2:1 to 4:1.7. The method according to claim 6 wherein the ratio of dilution fluidto production fluid in the mixture lies in the range 2.5:1 to 3.5:1. 8.The method according to any preceding claim wherein the step ofseparating (38) the dilution fluid (60) from the mixture involvesseparating a majority of the dilution fluid (60) from the mixture. 9.The method according to claim 8 wherein the step of separating thedilution fluid (60) from the mixture involves separating at least 90% ofthe dilution fluid (60) therefrom.
 10. The method according to anypreceding claim wherein the separation of the dilution fluid from themixture involves routing the mixture into a separator chamber (38) of asystem (14) at the remote location where separation of the dilutionfluid (60) from the production fluid (63) occurs as a consequence oftheir different specific gravities.
 11. The method according to claim 10wherein the system (14) at the remote location includes a pump (62) andthe conveying of the separated dilution fluid (60) to the dilution fluidaddition location (8) includes pumping the dilution fluid (60) with thepump (62).
 12. The method according to any preceding claim includingconveying some of the separated fluid (60) to a disposal well (78). 13.The method according to claim 12 wherein a pump (80) is situated at thedilution fluid addition location (8) and the step of conveying thedilution fluid (60) to the disposal well (78) includes pumping it therewith the pump (80).
 14. Apparatus for assisting flow of production fluidfrom a seabed wellhead (4) to a remote subsea location (14) includingdilution fluid addition means (8) situated close to or at the wellhead(4) for adding dilution fluid to the production fluid to produce amixture, a pipe (10) for conveying the mixture to the remote location(14), separating means (38) at the remote location (14) for separatingat least some of the dilution fluid (60) from the mixture and means (12,62 . . . ) for conveying the separated dilution fluid (60) from theremote location (14) to the dilution fluid addition means (8) foraddition to further production fluid flowing through the wellhead (4).